If hundreds of thousands of suns rose up at once into the sky, they might resemble the effulgence of the Supreme Person in that universal form.

PURPORT

What Arjuna saw was indescribable, yet Sanjaya is trying to give a mental picture of that great revelation to Dhrtarastra. Neither Sanjaya nor Dhrtarastra were present, but Sanjaya, by the grace of Vyasa, could see whatever happened. Thus he now compares the situation, as far as it can be understood, to an imaginable phenomenon (i.e. thousands of suns).